The Johnson wing of the Boston Public Library seriously needs some love. Designed by the architect Philip Johnson and opened in 1972, the monumental structure on Boylston Street — once heralded as a bold Modernist statement — has fallen in the public esteem, especially compared to the classical 1895 main library designed by Charles Follen McKim. Hulking, barren, and closed off from the street by a barricade of 97 vertical granite slabs, or plinths, the unwelcoming façade of the Johnson wing today mocks the spirit of the library’s inscribed motto, “Free to All.”

The interior of the building is little better. Once inside, library patrons are immediately confronted with a phalanx of metal detectors. The windows that peek above the granite slabs are tinted, adding to the gloom. Sightlines are blocked by huge stone walls; there is no hint of how one connects to the McKim wing. “The building is unloved because it doesn’t open itself up to the city,” says architect William Rawn, whose firm has been commissioned to fix what ails the Johnson. “Its stance is essentially anti-urban.”

The trouble is that the plinths, the interior granite walls, even the tinted glass — all features Rawn would remove — have designated landmark status, and any alterations need approval by the Boston Landmarks Commission. Johnson was one of the most significant designers of the 20th century; his plan for the new wing was a stripped-down, modern adaptation of the McKim building intended as a tribute, not a rebuke. So the overdue renovation of the building, crucial as it is, raises a thorny question: How does Boston, and cities like it, reconcile preserving important historical structures with the changing needs of the institutions housed within?

Amy Ryan, the BPL president, has already done much to bring the city’s libraries into the modern age. She’s as proud of the 20 languages spoken at the new East Boston branch — opened earlier this month and also designed by Rawn — as she is of the Copley branch. Her plan for the Johnson wing would double the size of the children’s library, create a new teen lab, upgrade the fiction shelves near the entry, add four bathrooms to the chronically underserved building, and explore the possibility of a café or bookstore opening to the street.

Drawings from Rawn’s shop show a bright, airy Johnson wing with two new entrances, landscaping, and clear connections inside to the McKim building and its beloved outdoor courtyard. The firm addresses alterations to Johnson’s building by honoring the architect’s original intent, gleaned from his writings and speeches: that public buildings should be places of grand open space, with a sense of anticipation and arrival.

The renovations would almost certainly boost access and attendance, and might also help enliven what has been a rather undistinguished commercial stretch of Boylston Street. This portends well for the full plan’s eventual approval. “The commission feels very strongly that the best way to preserve a historic building is to utilize it fully,” said Ellen Lipsey, executive director of Boston Landmarks Commission. “Landmarking is not about freezing anything.”

Still, only the first $16.1 million phase of the Johnson renovation has been approved in Mayor Thomas Menino’s budget. As a city department, the library has to hope that Marty Walsh, Boston’s next mayor, will continue Menino’s commitment to the library — branches and trunk alike — as a dynamic showcase for learning.

He should, because public libraries are enjoying a much-deserved renaissance. Contrary to conventional wisdom, a Pew Center report this summer found that the millennial generation (ages 16 to 29) was just as likely as older adults to visit a library; 65 percent of the age group had a library card. Soaring new public libraries have gone up in Seattle, Minneapolis, and Salt Lake City.

Providing the resources needed to deliver on the Johnson building’s potential is about something else, too. Especially for a new mayor known for his concern about social equity, the library represents a democratic ideal of free civic education, a true embodiment of the public realm. A refurbished Johnson wing with a large glass frontage, transparent and accessible, says Boston is not a defended city, a place afraid of opening itself up. Instead, it is a city turning its face to the future.